1. Field of the Invention
This invention relates to oscillators, and particularly to oscillators used to drive multiple switching regulators on a single integrated circuit (IC).
2. Description of the Related Art
Oscillator circuits are found in many different types of electronic circuitry. In some applications, two or more oscillators are employed, which may need to be at least occasionally synchronized.
For example, oscillators are found in the controller portions of most switching voltage regulator topologies. In some cases, multiple switching regulator controllers are provided on a single chip, with each controller having its own oscillator. Under some circumstances, it may be advantageous to have the operating frequencies of these multiple oscillators synchronized. For example, for a chip containing two switching regulator controllers arranged to use PWM to provide regulation, it is beneficial to have the controllers' ‘on’ pulses synchronized and out-of-phase, preferably by 180°. This can serve to reduce peak input current, as well as remove cross-talk interference between the two controllers, as both generate substantial noise on the chip's supply rails. However, at other times—under light load conditions, for example—the switching regulators might be arranged to enter a variable frequency mode, in which case the frequency synchronization should be terminated and the oscillators allowed to operate independently.
One type of oscillator employs an integrator which generates a ramping voltage, a comparator which determines when the ramping voltage crosses a predetermined threshold voltage, and a one-shot circuit that resets the integrator when the ramping voltage crosses the threshold. These types of oscillators can be particularly difficult to synchronize. For example, an asymmetrical master-slave arrangement can be employed; however, this can be problematic, as there is little that can be used to distinguish one switching regulator controller from the other. Also, oscillators tend to only be easily synchronized ‘up’ in frequency. This is because an external sync signal can early-terminate the ramp waveform; however, if the sync signal arrives after the internal reset signal, the reset signal has already done its work and it is too late. In addition, it can be difficult to determine which of the oscillators is at a higher frequency, in order to use it to ‘sync up’ the lower frequency oscillator. Another issue arises when it is desired to have the ‘on’ pulses synchronized and out-of-phase, preferably by 180° as discussed above, as syncing with an 180° phase shift requires some ‘memory’ of past events.